Introduction In adults, detection of any change in acoustic surroundings activates auditory cortices in temporal lobes and in frontal brain areas at around 100 ms and evokes an event-related potential (ERP) called N100. 1 Several neural processes elicit overlapping ERP components when the sound is novel, 2 when the sound is attended, 3 and even when there is a deviance in auditory input which is not consciously observed (mismatch negativity at 100–200 ms; MMN). 4 Any novel sensory input also elicits a non-specific vertex response in the same latency range. This cerebral response belongs to orienting reaction (OR), which redirects attention involuntarily to the new stimulus 5 and may set up a neuronal model of stimulus prop- erties with its first representation. 1,6 Thus, the devel- opment of a neuronal representation upon stimulus repetition should inhibit OR and abate corresponding ERP components. In addition, rapid repetition of identical stimuli diminishes the size of auditory ERPs, especially N100, due to refractory properties of eliciting neuronal populations. 7,8 In children, shortening of ERP latencies with age is usually taken as a sign of increased efficiency in receiving networks, 9 while changes in topographic distribution may indicate developmental changes in the mode of input processing. 10,11 Auditory ERPs, which are considered to represent similar cerebral activation as an ‘adult’ N100, emerge in early infancy at around 200 ms and the latency shortens until early adulthood. In a recent study of infants, spoken syllables evoked first ERP components over temporal brain areas at 200 ms and these decreased in size when the stimulus was repeated. 12 Responses at 400 ms diminished similarly but recovered completely with a change in stimulus. It was suggested that acoustic analysis was at the level of detection at 200 ms, whereas auditory features were recognized in less than 400 ms. We compared ERPs to elementary auditory input in school-aged children and in young adults during infrequent presentation of trains of identical tones. These trains of tones were separated by silent periods exceeding the putative duration of echoic mem- ory. 13,14 The aim of the study was to evaluate auto- matic processing of ‘novel’ auditory stimuli (i.e. the first tone in train) and refractory properties of cerebral responses during the very first repetitions. Since ERPs to frequent auditory stimulation change considerably during development, we expected to observe differential changes in the refractory behav- iour of ERP components due to different maturation rates of underlying neuronal populations. Materials and Methods We studied 42 healthy school-aged children (mean age 9 years ± 2 weeks) and 16 healthy young adults (mean age 28 years, range 23–49 years). All subjects gave their informed consent before the study. Tone pips (frequency 800 Hz; duration 85 ms) were delivered to the right ear in trains of four with an interstimulus interval (ISI) of 1 s and intertrain interval (ITI) of 12 s. Identical tones were delivered with a standard ISI of 1 s during MMN paradigm, in Neurophysiology, Basic and Clinical 1 1 1 1 1 p © Rapid Science Publishers Vol 8 No 6 14 April 1997 1327 WE compared event-related responses (ERPs) to non- attended frequent and intermittent auditory input in school-aged children and in young adults. In adults, both inputs elicited prominent auditory N100 responses at vertex. In children, intermittent stimulation evoked vertex responses with similar latency and refractoriness, whereas frequently delivered identical tones evoked responses on average at 240 ms. Sensitization of a sepa- rate neuronal population at 260–300 ms was obvious during intermittent stimulation in children. The dual behaviour, simultaneous ‘habituation’ of one neuronal population response and sensitization of another, may reflect the process of redirecting the attention and setting up a neuronal model. Furthermore, results sug- gest that a simplistic interpretation of developmental ERPs in which shortening of latencies represents matu- ration is insufficient. Key words: Auditory N100; Children; Development; Event-related potentials; Habituation Dual cerebral processing of elementary auditory input in children Jari Karhu, CA Eila Herrgård, 1 Ari Pääkkönen, Laila Luoma, 1 Eila Airaksinen 1 and Juhani Partanen Department of Clinical Neurophysiology and 1 Department Pediatrics, Division of Children’s Neurology, Kuopio University Hospital, Kuopio, Finland CA Corresponding Author NeuroReport 8, 1327–1330 (1997)